According to the charging document, a tipster called Mankato police on Monday and said he was at a friend's house around 3 a.m. Sunday when Shelley showed up wearing a torn red shirt. Shelley told the tipster he was downtown when he saw someone "starting something" with Nelson. Nelson, 20, and Shelley went to Mankato West High School together and were Facebook friends. (It appears one of them has defriended the other since Sunday morning -- either that or Nelson's account is private.)

Shelley told the tipster he "wound up and hit [the man messing with Nelson] as hard as he could," according to the charging document. (Nelson and his girlfriend both said they don't have any idea who the "man in red" was when questioned by authorities.)

Isaac Kolstad

A number of other tipsters got in touch with the Mankato PD and said they believe Shelley is the man shown in surveillance footage running from the scene of the altercation with a torn red shirt.

The force of the punch Shelley allegedly threw knocked Kolstad out and resulted in his head hitting the pavement with a sickening thud, witnesses say. Nelson then moved in and kicked Kolstad in the head once or twice, authorities say.

A Mayo Clinic doctor says Kolstad, a married 24-year-old father with a pregnant wife, has a skull fracture on the side of his head where Nelson allegedly kicked him. The former MSU-Mankato football player's injuries also include brain bleeding, swelling, and shifting, and deterioration of the lungs from lack of oxygen. His prognosis is grim.

Yesterday afternoon, a Minnesota Bureau of Criminal Apprehension special agent called Shelley. Shelly said he knew why the agent was calling, and explained he was leaving Mankato's Blue Bricks Bar and Eatery just after bar close Sunday morning when he saw a group including Nelson and his girlfriend arguing with a man in a black shirt.

Shelley said Kolstad then struck Nelson in the back of the head, causing him to fall to the ground. The man in the black shirt subsequently swung at Shelley, but missed. (It's unclear whether Kolstad is the same person man in the black shirt Shelley referred to.) Shelley told the BCA agent he left the scene, but not before the man in the black shirt grabbed him, ripping his shirt. He said he never hit Kolstad himself, and stood by that claim even after the agent told him the whole thing was filmed by a surveillance camera.

The mugshots of Shelley (left) and Nelson.

Despite his denial, Shelley was placed under arrest and charged today. The criminal complaint notes that the investigation is "ongoing."

"This complaint is prepared to comply with the court mandates of 36 and 48 hour rules [after which a suspect must be released if they aren't charged with a crime], and may be amended in the future, if appropriate," it says.